Linearity is an important performance characteristic of many electronic devices. The linearity of a device may be defined as the degree to which the amplitude (or power) level of an output signal of the device is directly proportional to the amplitude (or power) level of an input signal provided to the device.
The slope of a plot of a device's output signal level versus the device's input signal level may be referred to as the device's gain. For many types of devices, ideally the device should have a perfectly linear response wherein the output signal level changes perfectly linearly in response to a change in the input signal level, yielding a constant gain.
However actual devices do not have perfectly linear responses, and thus there is some variation in the gain of the device as a function of one or more parameters. In many cases, it is desired to be able to determine and specify the linearity of a device as a function of some parameter (e.g., input signal level, frequency, input power level, temperature, input voltage level, etc.). Linearity may be specified in a number of different manners, but one common way to specify linearity is to identify the deviation of the device's actual output-signal-level-versus-input-signal-level response from a straight line over a given range of a particular parameter. Where a device's gain, G, is specified in decibels (dB), the device's linearity may be specified as the amount in dB that the actual output-signal-level-versus-input-signal-level varies from G. To illustrate, when an example amplifier has a nominal gain of 20 dB, the amplifier's linearity may be specified, for example, as “±0.50 db over an input signal level range from −80 dbm to −20 dbm.” This is just one example manner of specifying a device's linearity which is provided for illustration purposes, and many other ways of specifying linearity are known to those of skill in the art.
One category of devices for which linearity may be an important performance characteristic includes detectors. As broadly defined here, a detector is a device which receives a receive signal and which, in response to the receive signal, outputs an output signal whose signal level may linearly track the signal level of the input signal. Examples of detectors include, without limitation, crystal detectors, diode detectors, amplifiers, mixers, down-converters, analog-to-digital converters (ADCs), and power meters.
FIG. 1A illustrates an arrangement 10 for measuring the linearity of a device-under-test (DUT) 50. The arrangement 10 includes a signal generator 15, a reference attenuator 20 and a processor 40. Of significance, in arrangement 10 reference attenuator 20 has a calibrated or otherwise predetermined attenuation characteristic, and attenuation characteristic data for reference attenuator 20 is available to processor 40 (e.g., stored in a memory accessible by processor 40). In some alternative arrangements, reference attenuator 20 could be replaced by a reference amplifier or other variable gain device with a calibrated or otherwise predetermined gain characteristic.
Arrangement 10 may be employed to determine the linearity of DUT 50 by changing an attenuation value of reference attenuator 20 to thereby change the power level supplied to DUT 50. Processor 40 may then compare the corresponding change in the output signal level produced by DUT 50 to the change in attenuation provided by reference attenuator 20 based on the attenuation characteristic data for reference attenuator 20. This measurement may be repeated for a number of different attenuation values in order to determine the linearity of DUT 50.
FIG. 1B illustrates another arrangement 11 for measuring the linearity of a DUT 50 that does not require a calibrated reference attenuator. The arrangement 11 includes a signal generator 15, an attenuator 25, a power splitter 30, a linear reference detector 35, and a processor 40. Of significance, in arrangement 11 linear reference detector 35 has a calibrated or otherwise predetermined linearity characteristic, and the linearity characteristic data for linear reference detector 35 is available to processor 40 (e.g., stored in a memory accessible by processor 40).
Arrangement 11 may be employed to determine the linearity of DUT 50 by changing an attenuation value of attenuator 25 to thereby change the power level supplied to both DUT 50 and linear reference detector 35 by the same amount. Processor 40 may then compare the corresponding changes in the output signal levels produced by DUT 50 and linear reference detector 35 in response to the change in their input signal levels, together with the linearity characteristic data for linear reference detector 35, in order to determine the linearity of DUT 50. In arrangement 11, attenuator 25 does not need to be calibrated or have a known attenuation response because whatever the actual attenuation change it provides between two different attenuator settings, the same attenuation in signal level is provided to both DUT 50 and linear reference detector 35.
The arrangements and techniques described above have some drawbacks, notably with respect to speed and accuracy. For example, in some cases, linearity tests must be able to determine the linearity of devices which employ high speed (e.g., 10 megasamples/second) analog-to-digital converters with more than 14 bits of resolution with quantization error reduction. In these cases, a very high degree of speed and accuracy is required. However, linear reference detectors such as that employed in arrangement 11 commonly require long measurement settling times. Also, arrangements 10 and 11 both depend upon the use of characteristic data for a calibrated or reference device (e.g., reference attenuator 20 or linear reference detector 35). So any change in this behavior after the reference device has been calibrated or characterized, or any error in characterizing the reference device, results in an inaccuracy or uncertainty in the linearity measurement of DUT 50.
Accordingly, it would be desirable to provide a method and system for determining the linearity of a device under test which does not depend upon having a calibrated reference attenuator or detector. It would also be desirable to provide a method and system for determining the linearity of a device under test which can avoid the long settling times associated with the use of linear reference detectors.